Let's analyze the pattern in the three-letter groups.
First Letter: D(4), F(6), H(8), \(\underline{\hspace{0cm}}\), L(12). This is an arithmetic progression with a common difference of +2. The missing letter is 8 + 2 = 10, which is J.
Second and Third Letters: The second and third letters are identical in each group and follow a pattern.
N(14), Q(17), T(20), \(\underline{\hspace{0cm}}\), Z(26).
Let's find the differences:
17 - 14 = +3
20 - 17 = +3
The pattern is an arithmetic progression with a common difference of +3.
The missing letter should be 20 + 3 = 23, which is W.
The next two letters should be WW.
Let's check if this fits the last term: 23 + 3 = 26, which is Z. It fits perfectly.
Combining the patterns, the missing group is JWW.